This relates generally to bias circuitry and, more particularly, to bias circuitry for amplifiers.
Integrated circuits such as image sensors may contain amplification circuitry such as high-speed drivers. In high-speed drivers, at least one amplification stage is used to drive a signal on an output line. Push-pull amplifiers may be used as one of the amplification stages (e.g. class B push-pull amplifiers or class AB push-pull amplifiers).
In conventional push-pull amplifiers, a first output transistor is used to sink current from an output load to a ground power supply terminal, while a second output transistor is used to supply current to the output load from a positive power supply terminal. Typically, when an input voltage signal is applied to the input terminal of a conventional push-pull amplifier, the output terminal may either be dissipated of current by the first output transistor or supplied current by the second output transistor. Ideally, the first output transistor is never sinking current when the second output transistor is supplying current. However, this operating behavior may require that the transistors be biased below or at their threshold voltages (e.g., the transistors are biased such that, at zero input voltage, both transistors are off). Due to integrated circuit delay and the exponential current characteristics of transistor behavior at bias voltages near the transistor threshold voltage, the transition between the first output transistor sinking current and the second output transistor supplying current may not be synchronized (e.g., when the first output transistor stops sinking current, the second output transistor may not immediately begin supplying current). The effect on the push-pull amplifier output due to lack of synchronization between the two output transistors may be referred to as crossover distortion.
To counteract the effects of crossover distortion, the output transistors of push-pull class B amplifiers may be biased at voltages above the transistor threshold voltage. Such a configuration may be referred to as a push-pull class AB amplifier. In a class AB amplifier, the current through both of the output transistors may be non-zero when no input voltage signal is applied to the amplifier (e.g., at the given bias voltages, the first output transistor may be sinking current while the second output transistor is supplying current). The current through the first output transistor and the second output transistor while no input voltage is applied to the amplifier may be referred to as quiescent current.
Integrated circuits may experience undesirable effects during manufacturing and normal usage. These effects may include process, voltage, and temperature variations (so-called PVT variations). PVT variations may further cause variations in supply voltage, bias voltages, resistor values, transistor threshold voltages, and other factors affecting the amplifier quiescent current. The amplifier quiescent current may increase as a result of PVT variations (e.g., if the supply voltage increases, the quiescent current through the output transistors will increase). An increase in quiescent current may result in unacceptable circuit power consumption.
In an arrangement where push-pull amplifiers are used to drive an output signal, direct current may be generated from the supply voltage terminal to the ground terminal when an input signal to a class AB amplifier transitions between a high and a low voltage. During this transition, the first output transistor and second output transistor may both be activated for a period of time (e.g., when the input signal voltage level transitions near the bias voltage level, the first output transistor may be sinking current while the second output transistor is supplying current). The current generated between the positive power supply line and power supply ground may be referred to as shoot-through current. Conventional push-pull amplifiers may have high shoot-through currents that result in unacceptable amplifier power consumption.
Therefore, it would be desirable to design an amplifier with low quiescent current and low shoot-through current.